API Testing Complete Guide 2026 — REST, GraphQL, Contract & Beyond

Last updated 2026-07-20 · 17 min read · By Avinash K

API testing is the highest-leverage layer in the pyramid — an API test runs in tens of milliseconds and catches the same bug a 30-second UI test would. This guide covers everything from your first request to production-grade contract testing.

• Methods: GET (read), POST (create), PUT (replace), PATCH (update), DELETE (remove).
• Status codes: 2xx success, 3xx redirect, 4xx client error, 5xx server error.
• Headers: Authorization, Content-Type, Accept, Idempotency-Key.
• Idempotency: GET/PUT/DELETE must be idempotent; POST usually is not.

Postman remains the fastest way to explore an API. Turn requests into a Collection, add test scripts (pm.test), run in CI with Newman. Deep dive: Postman API testing tutorial. Migrating to code? Use our Postman → Code Converter.

given()
  .header("Authorization", "Bearer " + token)
  .queryParam("status", "shipped")
.when()
  .get("/orders")
.then()
  .statusCode(200)
  .body("orders.size()", greaterThan(0))
  .body("orders[0].id", notNullValue());

Single endpoint, POST-only, schema-first. Test the schema itself (introspection), query shape, error semantics, and N+1 performance. Use Apollo's @defer/@stream in 2026 apps.

Microservice teams that add Pact see integration-defect escape rate drop by 60-80%. Consumer defines expected interactions, producer verifies them in CI. The Pact docs are the canonical reference.

The OWASP API Security Top 10 (2023, still current) covers 90% of issues: broken object-level auth, broken function-level auth, injection, mass assignment, security misconfiguration. Automate the checks you can (auth on every endpoint) and manually test the rest.

import http from 'k6/http';
import { sleep, check } from 'k6';
export const options = { vus: 50, duration: '2m' };
export default function () {
  const res = http.get('https://api.example.com/orders');
  check(res, { 'status 200': (r) => r.status === 200 });
  sleep(1);
}

Comparison: k6 vs JMeter.

Practice the exact questions asked at product companies in our API testing interview Q&A. Then rehearse live with the AI mock interview.

Testing a single API is straightforward; testing forty that call each other is not. Continue with our microservices testing strategy for the layered approach — contract, component, and end-to-end — that keeps distributed suites fast.